The present invention relates generally to a technique for implanting a prosthesis and more particularly to a technique for implanting a prosthesis of the "cementless" type.
Since the development of the first cemented hip arthroplasty during the 1960s, cemented prosthesis arthroplasty has remained a successful orthopedic procedure for the treatment of arthritis and other afflictions of joints. The fixation of early devices was achieved by using methyl methacrylate which is a polymer created by mixing a powder and liquid. After mixing, the methyl methacrylate goes through several phases until it achieves a hardened state. Initially the material is free-flowing, then becomes doughy, and finally the material hardens at the conclusion of the polymerization process. The polymerization of methyl methacrylate is an exothermic reaction reaching a temperature of approximately 54.degree. C.
Methyl methacrylate, also known as bone cement, has no bonding properties. It simply acts as a non-bioabsorbable spacer that fills the bone cavity, thus providing for prosthesis fixation. The problem with cemented arthroplasty is the lack of long term durability. There is a significant incidence of loosening of these devices caused by failure at the bone cement interface. This is especially true when these devices are implanted in younger patients. This failure rate has spurned an interest in cementless prosthesis arthroplasty.
Another problem arises because the polymerization of methyl methacrylate is an exothermic reaction. The reaction itself destroys some of the tissue surrounding the prosthesis.
Cementless prosthesis technology was developed during the 1970's and 80's, and consists of implantation of devices that have a porous surface. Such prostheses are disclosed, for example, in U.S. Pat. No. 3,605,123 (Hahn), which is incorporated herein by reference. The prosthesis disclosed in this reference includes a dense metal base and an overlying highly porous metallic layer which permits growth of bone tissue into the pores. Other cementless prostheses are disclosed in U.S. Pat. Nos. 3,855,638 (Pilliar) and 4,550,448 (Kenna), which are also incorporated herein by reference. Cementless prostheses are available commercially from numerous companies, including Zimmer, Inc., Warsaw, Ind.; Johnson & Johnson, New Brunswick, N.J.; and Dow Corning, Lexington Tenn.
The porous surface of the implants permits bone ingrowth into the device, providing for prosthesis fixation. The requirements for bone ingrowth into a porous surface include rigid internal fixation, proper pore size, usually between 250 and 400 microns, and intimate contact between the prosthesis and the host bone. If any of these requirements are not met; poor fixation is inevitable, usually with fibrous tissue ingrowth rather than bone.
There have been recent successful clinical trials with the use of cementless prosthesis for the hip and knee and many surgeons believe that this is the best surgical procedure for young patients where the durability of a cemented prosthesis is clearly in question. Additionally, cementless devices are almost always used in revision surgery.
Although techniques for implanting the cementless devices are known in the art, these techniques are not without disadvantages. The primary problem with the use of cementless devices in a human bone is achieving rigid fixation and intimate contact between the device and host bone. The device is typically press fit into the prepared bone surfaces, either the medullary canal of the femur and acetabulum (in hip arthoplasty) or the surfaces of the distal femur and proximal tibia (in total knee arthroplasty). The fit needs to be so tight and exact that fracture of bone during insertion is not an uncommon occurrence.
A second problem encountered is the wide variation in the size and shape of human bone. Despite the available selection of sizes of these prostheses, it is impossible to perfectly fit the prosthesis to the host bone.
A further problem is in the bone preparation, even with the most precise surgical cuts and techniques, microscopic gaps between the host bone and the prosthesis are inevitable, making bone ingrowth unpredictable. This problem is of particular concern in revision surgery. After each arthroplasty, marked bone loss occurs creating large holes where it is exceedingly difficult to achieve fixation and intimate contact between the porous surface and the host bone. In revision surgery bone ingrowth into the porous surface is very unpredictable.
It is therefore desirable to eliminate gaps between the prosthesis and the host bone. A method of eliminating this gap, thus providing for intimate contact would be advantageous for bone ingrowth. Selection of the appropriate material to fill the gap is critical to the success of such a method. The most desirable material would be one that temporarily fills the gap, provides for initial fixation, is osteoconductive, and is bioresorbable, allowing bone to replace this material. In addition, it must be well tolerated by the host.
A material that can be used in the operating room that has similar properties to methyl methacrylate is also desirable. Methyl methacrylate goes through stages of first being free-flowing, then doughy and finally polymerizes into a hardened material. Such a material will in effect result in a biologic bone scaffold or interface which can be used, for example, in conjunction with a cementless hip without any change in the surgical technique, other than the use of the interface prior to seating the device.
Finally, the material needs to have a controlled rate of both hardening and resorbtion such that it will harden in the operating room short of 10 minutes, and will last a minimum of four weeks in the host to achieve prosthesis fixation.
While the prior art discloses the use of a bioresorbable material for use in bone implantations and prosthesis parts, there has been no suitable solution to the gap-filling problem described above. Most of the materials which have been tested are either inorganic or organic polymers, many of which are used in the dental industry.
For example, U.S. Pat. No. 4,619,655 (Hanker et al.) discloses prostheses for bone repair or reconstruction which are comprised of plaster of Paris mixed with a non-bioresorbable calcium particles. The plaster of Paris functions as a biodegradable scaffold or binder to hold the non-bioresorbable calcium particles together. This patent teaches that the plaster of Paris is absorbed and replaced simultaneously by fibrovascular tissue which in turn is replaced by bone. The prostheses are, however, comprised of the mixture of the non-bioresorbable calcium particles, with the plaster of Paris merely holding the particles together. The mechanical strength of the calcium particle matrix after the plaster of Paris has been resorbed may be unsatisfactory for a prosthesis.
U.S. Pat. No. 4,356,572 (Guillemin et al.) discloses the use of calcium carbonate as a bone prosthesis part. According to one embodiment disclosed in this patent, a nonresorbable endoprosthetic element includes a hollow section which contains calcareous material. The endoprosthetic element is configured such that the hollow part communicates with the outside of the element. When the prosthesis is inserted, the communicating sections of the prosthesis are disposed such that they are in contact with bony substance. The calcium carbonate is progressively replaced by newly formed bone, forming an anchorage for the endoprosthetic element.
U.S. Pat. No. 4,309,488 (Heide et al.) discloses bone replacement materials that consist of a solid core of metal that is compatible with body tissue and a calcium phosphate ceramic material disposed on the peripheral areas of the core. The calcium phosphate ceramic material serves as an interface between the core and the bone tissue.
These prior art references, however, do not provide a solution to the problem of the unpredictability of the bone ingrowth in the gaps which result between the host bone and the prosthesis.
Some efforts have also been devoted to finding a more suitable bone cement. For example, U.S. Pat. No. 4,373,217 (Draenert) discloses the use of a resorbable tricalcium phosphate in a methyl methacrylate implantation material. The implantation material disclosed by this reference however suffers from the disadvantages described above with respect to the use of methyl methacrylate.
Therefore in view of the above it is a primary object of the present invention to provide a method of implanting a cementless prosthesis which will achieve rigid fixation of the prosthesis.
It is a further object of the present invention to provide a method of implanting a cementless prosthesis which will improve bone ingrowth in gaps which result between the prosthesis and the host bone.
It is a more specific object of the present invention to provide a method of implanting a prosthesis wherein gaps which result between the prosthesis and the host bone are initially filled to provide an initial fixation and wherein the filling material is subsequent1y replaced by bone tissue.